1. Field of the Invention
This invention relates to a rotational angle detector suitable for use in an ignition system for an internal combustion engine.
2. Description of the Related Art
FIG. 1 schematically shows a conventional rotational angle detector in which a disk 2 is fixed to a shaft 1 which is rotated in synchronism with the rotation of an internal combustion engine. A light emitting element 3 and a light receiving element 4 are disposed in opposed positions with the disk 2 interposed therebetween. The light emitting element 3 and the light receiving elements 4 are mounted in respective box-like holders 5 and 6 formed of a plastic each having a front opening at which a plate 7 or 8 having slits is disposed. The slits of the plates 7 and 8 are aligned with each other.
The thus-constructed rotational angle detector is disposed in an ignition distributor of an internal combustion engine. As the disk 2 is rotated with the rotation of the shaft 1 based on the rotation of the engine, light emitted from the light emitting element 3 is intermittently introduced into the light receiving element 4. A signal outputted from the light receiving element 4 is processed in a processing circuit (not shown) in order to detect the angle of rotation of the shaft 1. The detected angle is used as a reference angle during engine ignition timing control.
During use of this conventional rotational angle detector in the ignition distributor, there exist a possibility of a high voltage being induced in the shaft and the disk 2 due to a discharge from a distributor rotor electrode and, hence, the possibility of the induction of this induced voltage in the signal processing circuit of the rotational angle detector, resulting in a malfunction of the signal processing circuit. To cope with this problem, a shield (e.g., shielding plates 15) for shutting off electromagnetic induction, i.e., voltage induction from the shaft 1 and the disk 2 to the signal processing circuit is ordinarily provided outside the rotational angle detector.
However, the gap between the slit plates in this specific construction of the detector is small and it is not possible to provide a shielding plate at this position. In addition, the slit plates 7 and 8, which are fixed to the plastic holders 5 and 6, are electrically floating and it is therefore difficult to remove electromagnetic charge thereon, which may cause noise by entering via this position.